It relates in particular to the final purification of biogas, with the aim of producing methane, preferably liquid methane; this is because liquefaction is a method of packaging methane which makes it possible to economically store it and transport it.
Anaerobic (oxygen-free) fermentation of organic waste gives off a gas essentially consisting of methane and carbon dioxide, known as biogas. This process is developing fast, both in order to limit the emissions of greenhouse gases into the atmosphere but also to make use of the biogas thus produced, which is an appreciable energy source.
Biogas is intended in particular to feed electrical turbines or to act as fuel for vehicles.
Resulting from anaerobic fermentation, biogas comprises CO2 and CH4 in respective proportions depending on the nature of the materials fermented; in general, the biogas produced comprises between 55% and 65% of methane.
Biogas, once purified from its carbon dioxide, from its water and from its hydrogen sulphide H2S, can be made use of economically as methane, in particular as fuel.
As recalled above, liquefaction is a preferred method of packaging methane, whether for storage purposes or transportation purposes. Any unit for the purification of CO2-comprising biogas will thus require the inclusion, in its process, of a final purification of methane in order to remove, among other constituents, those which are incompatible with the liquefaction or another treatment requiring a change to cryogenic temperatures. It will be advisable in particular to limit the CO2 concentration to a maximum content in fine of less than 100 ppm.
The final purification of methane resulting from biogas with the aim of producing liquid methane involves different processes known from the state of the art, which are pressure swing adsorption (PSA), temperature swing adsorption (TSA) or washing with amines.
These normal adsorption techniques involve, for the regeneration of adsorbents, large amounts of gas. However, the production sites for biogas (digester, landfill sites, and the like) generally do not have available large amounts of clean gases (pure CH4, N2) for the regeneration of the adsorbers.
Furthermore, when the amount of CO2 in the biogas is significant (>1%), the exothermicity of the adsorption heats up the adsorbent, thus damaging its adsorption capacity, and it is thus essential to have available efficient cooling; during the regeneration of the adsorbent, it is necessary, on the contrary, to contribute a large amount of heat for the desorption of the impurities.
The circulation in a closed loop with reheating during the regeneration phase is not effective as the circulating gas very rapidly becomes loaded with impurities (CO2) and distributes the impurities over the whole of the adsorbent bed. The residual content of impurities is then too high to achieve a gas quality compatible with liquefaction (<100 ppm). A flushing gas is necessary.
In order to be able to produce purified methane while operating the plant continuously, use is made, in a known way, of two adsorbers in parallel, one being in the adsorption phase while the other is in the desorption phase.
There is known, from US 2008/0289497, a system for purifying methane for the purpose of liquefying it, and in particular for removing CO2, using three adsorbents. While one is in the adsorption phase, the second is in the desorption phase and the third is cooled, the presence of three adsorbers making possible heat transfers.
While the above system makes it possible to limit the external energy contribution, it requires, however, the use of three adsorbers in parallel, which generates additional costs in comparison with a conventional plant using two adsorbers. The problem which is posed is thus that of providing a solution for purifying impure methane, in particular resulting from biogas, so as to produce methane having a purity compatible with liquefaction, while limiting the costs, both in terms of capital costs and in terms of operating costs—no third adsorber, reduced consumption of utilities, in particular of energy.
The term “impure methane” (or “flow rich in methane”) is understood to mean methane having a CO2 content of less than 5%, preferably of less than 2%.
The term “purified methane” (or “methane having a purity compatible with liquefaction”) is understood to mean, according to the invention, methane exhibiting a carbon dioxide content of less than 100 ppm, preferably less than 50 ppm.